1. Field of the Invention
The present invention relates to methods for preparing amides, and more particularly, to a method for separating an amide from a ketoxime.
2. Description of the Prior Art
Caprolactam is an important raw material in the manufacture of nylon 6 fibers and thin films. Beckman rearrangement of cyclohexanone oxime is an important reaction step in producing caprolactam. Currently, oleum is used as a catalyst for converting cyclohexanone oxime to caprolactam sulfate during Beckman rearrangement, and then ammonia is used for neutralization, so as to obtain caprolactam. While the conversion rate of cyclohexanone oxime is almost 100% and the selectivity for caprolactam is 99%, a large amount of low-valued ammonium sulfate is generated during the reaction, and concentrated sulfuric acid used for catalysis causes problems such as corrosion to the whole equipment and environmental pollution. In the recent years, researches on new production technologies of caprolactam focus on reducing or avoiding the generation of the by-product, ammonium sulfate. U.S. Pat. No. 6,265,574 discloses that the molecular sieve is used as the catalyst for a gaseous phase Beckmann rearrangement reaction in a fluidized bed system. In this method, no by-product, ammonium sulfate, is formed in the gaseous reaction; however, the selectivity of caprolactam is only 95.7%, and the operation temperature (300 to 350° C.) is higher than that of a liquid phase reaction. In addition, the catalyst in the gaseous reaction is easily inactive, and the regeneration is frequently performed, such that the catalyst cannot be used for the long term operation.
Moreover, in comparison with the gaseous phase reaction, the liquid-phase rearrangement has advantages including moderate reaction conditions, fewer requirements to the equipments, etc., and is advantageous to the reconstruction of the current equipments. As a result, scholars worldwide have put efforts on developing liquid-phase rearrangement, and attained substantial developments and breakthrough. For example, in Chinese Patent No. 1852898A assigned to Sumitomo Chemical Company Ltd. in Japan, an ionic liquid having the sulfonate group is used as a catalyst to obtain the selectivity of caprolactam up to 99%. In Chinese Patent No. 1919834 assigned to Lanzhou Institute of Chemical Physics in China, an ionic liquid having sulfuryl chloride is used as a catalyst to obtain the selectivity of caprolactam up to 97.2%. In WO2008/145312A1 assigned to DSM N.V. in Netherlands, an anionic solution having sulfate is used for conversion reaction to obtain the selectivity of amide up to 99%.
However, the above-mentioned catalysts cannot be regenerated for multiple times. The water content of the catalyst composition is too high, and the cyclohexanone oxime is hydrolyzed to cyclohexanone, such that the selectivity is decreased. The water needs to be removed before the regeneration of the catalyst composition.
Hence, there is a need to develop a catalyst composition for enhancing the tolerance to water, the conversion rate of cyclohhexanone oxime and the selectivity of the amide in the preparation of an amide.